In coronary arteries, vascular diseases including vessel lumen narrowing, usually due to atherosclerotic plaque, can lead to reduced blood flow to a heart muscle, angina (chest pain) and myocardial infarction—a heart attack. A variety of interventional treatments of cardiovascular disease are presently available to identify and treat such narrowing of a vessel lumen. Examples of such treatments include balloon angioplasty and/or deployment of stents. Diagnostic imaging is utilized to identify the extent and/or type of blockages within vessels prior to and/or during the treatment of such blockages. Diagnostic imaging enables doctors to ensure proper treatment of diseased vessels and verify the efficacy of such treatment.
In general, two distinct manners exist for generating diagnostic images for the identification and treatment of cardiovascular disease within a vasculature. A first manner of diagnostic imaging involves generating a radiological image of a stream flowing through a blood vessel's lumen from outside the vessel lumen. The purpose of generating an image of such flow is to identify blockages within diseased blood vessels that restrict blood flow. The extent of a vessel's lumen is traditionally imaged using angiography, which involves rendering a two-dimensional view of one or more vessels within a portion of a patient's vasculature through which radiopaque contrast media has been injected. The two-dimensional angiographic image can also be viewed real time by fluoroscopy. During such procedures, the images are potentially captured in various digital media, or in cine angiography (cine). Cine angiography, though rendering higher quality images of blood vessel lumens, exposes patients to high levels of ionizing radiation.
Fluoroscopy, generally using substantially less intense radiation than angiography, is used by physicians primarily to visually guide diagnostic and therapeutic catheters or guidewires, including one or more radiopaque markers, through vessels. The radiation intensity during fluoroscopy is typically one-tenth the intensity of radiation to which a patient is exposed during cine angiography. Many catheters have radiopaque markers that are viewable on a fluoroscope, thereby enabling a physician to track the location/path of such catheters as they are inserted within and/or withdrawn from patients. The platinum spring coil of guidewires also serves as a radiopaque marker. The lower radiation intensity of fluoroscopy allows a greater duration of use during a diagnostic/treatment procedure. However, due to its greater time of use, the total radiation exposure from fluoroscopy during an interventional treatment procedure can greatly exceed the radiation exposure during a typical cine angiography procedure. Thus, it is incumbent upon a physician to minimize the duration of time that a fluoroscope is used during a diagnostic and/or interventional treatment procedure.
The first manner of imaging, described above, has a number of drawbacks. For example, limited flow of contrast media near vessel walls and extreme variations in vessel cross-sections can result in incomplete filling of the vessel with a sufficient concentration of contrast media. As a consequence, the diameters of vessel segments can be misrepresented in an angiographic image. For example, a left main coronary artery cross-section is often underestimated by angiography. This can be problematic when attempting to judge the significance of a blockage within the vessel or when choosing the size of the treatment balloon or stent. An under-sized balloon or stent will not provide as effective treatment as a properly sized device. Furthermore, in angiography, a vessel's cross-section is determined by a two-dimensional view which may not accurately represent an actual extent of blood vessel narrowing.
Furthermore, to achieve an optimum treatment result, it is important to correctly determine a true target diameter of a native blood vessel—the diameter of a non-diseased blood vessel. However, angiography is ineffective in determining the target diameter of a vessel with disease along its entire length. For example, since vessels tend to taper in diameter along their length, a uniformly narrowed vessel may appear normal in an angiographic image.
Finally, angiography does not facilitate differentiating between different types of tissue found in atherosclerotic plaque. For example, in coronary arteries prone to producing a heart attack, necrotic tissue is thought to be more prevalent than purely fibrous tissue. Thus, while providing a good way to identify severe blockages, angiography is not always the best diagnostic imaging tool due to the incomplete nature of the angiographic image data.
The second manner of intravascular imaging comprises imaging the vessel itself using a catheter-mounted intravascular probe. Intravascular imaging of blood vessels provides a variety of information about the vessel including: the cross-section of the lumen, the thickness of deposits on a vessel wall, the diameter of the non-diseased portion of a vessel, the length of diseased sections, and the makeup of the atherosclerotic plaque on the wall of the vessel.
Several types of catheter systems have been designed to track through a vasculature to image atherosclerotic plaque deposits on vessel walls. These advanced imaging modalities include, but are not limited to, intravascular ultrasound (IVUS) catheters, magnetic resonance imaging (MRI) catheters and optical coherence tomography (OCT) catheters. In addition, thermography catheters and palpography catheters have also been demonstrated to generate vessel image data via intravascular probes. Other catheter modalities that have been proposed include infrared or near-infrared imaging.
In operation, these intravascular catheter-mounted probes are moved along a vessel in the region where imaging is desired. As the probe passes through an area of interest, sets of image data are obtained that, correspond to a series of “slices” or cross-sections of the vessel, the lumen, and surrounding tissue. As noted above, the catheters include radiopaque markers. Such markers are generally positioned near a distal catheter tip. Therefore, the approximate location of the imaging probe can be discerned by observing the catheterization procedure on either a fluoroscope or angiographic image. Typically imaging catheters are connected to a dedicated console, including specialized signal processing hardware and software, and display. The raw image data is received by the console, processed to render an image including features of concern, and rendered on the dedicated display device.
For example, IVUS images used to diagnose/treat vascular disease generally comprise sets of cross-sectional image “slices” of a vessel. A grayscale cross-sectional slice image is rendered, at each of a set of positions along the vessel based upon the intensity of ultrasound echoes received by an imaging probe. Calcium or stent struts, which produce relatively strong echoes, are seen as a lighter shade of gray. Blood or vessel laminae, which produce weaker echoes, are seen as a darker shade of gray.
Atherosclerotic tissue is identified as being the portion of a cross-sectional image between an internal elastic lamina (IEL) and an external elastic lamina (EEL). The ability to see the vessel lumen, and calculate its dimensions, allows the diameters and cross-sectional area of the vessel to be determined more reliably than the limited two-dimensional angiography. Because IVUS does not rely upon dispersing a contrast agent, IVUS is especially useful in generating images of the left main coronary artery as described above. Furthermore, the ability to view the EEL, and calculate its dimensions, allows an IVUS image to render a more reliable determination than angiography, of the correct diameter and length of the balloon or stent to use when restoring proper blood flow to a blocked/diseased vessel. Advanced IVUS images have also been described which perform tissue characterization and denote different types of tissue with a color code. One such modality is described in Vince, U.S. Pat. No. 6,200,268. Like IVUS, the other catheters mentioned above display a series of cross-sectional images from which additional information can be obtained.
Catheter-mounted probes, and in particular, IVUS probes can be configured to render a variety of two and three-dimensional images. In addition to the two-dimensional transverse cross-sectional images discussed above, a longitudinal planar image can be constructed from a plane which cuts through a “stack” of cross-section “slices”. In addition, three-dimensional “fly-through” images can be constructed from information in a series of cross-sectional slices of a vessel. Though these three-dimensional images can be visually impressive, the two dimensional angiography image remains the primary basis for determining the location of a catheter in a vessel, and the “schematic” reference through which the physician plans and carries out a treatment procedure.
In creating the “stack” or “flythrough” images, some assumptions are made by image data processing software in terms of the orientation of each slice to the next. In many cases the compound images, rendered from a series of transverse cross-sectional slices, are rendered in the form of a straight vessel segment. In reality, vessels can curve significantly. In segment visualizations that render straight segments, spatial orientation of each cross-sectional slice in relation to other slices is not measured. In addition, the rotational orientation of a catheter-mounted probe is generally not known due to twisting of the catheter as it passes through a vessel. Therefore, the angular relation between adjacent slices is not generally known. In many cases, these limitations do not significantly effect treatment of a diseased vessel because the typical treatment modalities (balloons, stents) are not circumferentially specific. A balloon, for example, dilates a vessel 360° around a lumen.
In view of the advantages provided by the two above described methods of imaging vessels, many catheter labs use both methods simultaneously to diagnose and treat a patient. However, an angiographic image provided on a different display monitor than a corresponding IVUS image (or the other image rendered by a catheter-mounted probe), presents challenges to a obtaining a comprehensive understanding of a state of a diseased vessel. For example, a physician identifies specific structures (e.g. feeder vessels) in cross-sectional images in order to determine a location on a vessel presented on an angiography display that needs to be treated. Coordinating images rendered by two distinct display devices can become cumbersome as the physician refers back and forth between two different screens on two distinct display devices. In addition, when a video loop of IVUS images is recorded, to be played back later on a machine, a corresponding angiographic image is not recorded in sync with it. Therefore, during playback, the specific cross-section being viewed needs to be compared to the vessel angiography, which is usually on a separate file.
A known visualization display simultaneously provides an angiogram, an IVUS transverse plane view, and an IVUS longitudinal plane view. A red dot is placed upon the angiogram corresponding to a currently displayed IVUS transverse plane view. A blue line is placed upon the angiogram corresponding to a currently displayed longitudinal plane view. The reference dot and line are only as valuable as the accuracy of the process that registers their positions on the angiogram.